Familiar ground in corn weed control

Cotton farmers who are switching to corn this spring should feel somewhat at home when it comes to controlling weeds, a University of Missouri Extension weed scientist says.

Kevin Bradley, based at the University of Missouri’s Columbia campus, says he suspects some of the same precepts that hold sway in corn would also be true for cotton.

“Cotton producers who are going to corn should feel pretty confident in their weed control because many of the same rules apply,” he said. “You need to stay clean and start with a pre-emergence herbicide. Corn is much more susceptible than soybeans to early season weed competition.”

That could be especially true in Roundup Ready corn, which more Mid-South corn producers have been planting to try to shield the crop from increasing problems with glyphosate drift.

With corn futures topping $4 a bushel, Missouri farmers are also expected to plant more corn in 2007, although soybeans probably will remain the dominant crop. (The state typically has 5 million acres of soybeans and 3 million acres of corn.)

Bradley has been urging growers switching to corn, especially Roundup Ready corn, to consider changes to the weed control strategy they employ in Roundup Ready soybeans.

“I don’t think it will be wise for growers to conduct weed management practices in Roundup Ready corn as in Roundup Ready soybeans,” he said. “Research indicates corn is much more susceptible to early season yield loss than soybeans. We can’t wait until weeds are 6 inches tall or taller to remove them.”

Bradley says his predecessor, Bill Johnson, long-time weed scientist at the University of Missouri, found that in 12 out of 13 years, a “two-pass” program consisting of a pre-emergence herbicide followed by a postemergence herbicide provided the best overall weed control in July. That study involved more than 2,000 observations statewide.

Bradley decided to revisit the subject and compared three program approaches in 29 trials conducted in Missouri over the last five years.

In 19 of the 29 trials, the highest corn yields were obtained with a two-pass program consisting of a pre-emergence herbicide followed by a postemergence herbicide. In eight of the trials, a one-pass postemergence program that also contained a residual herbicide provided the highest corn yields.

“Collectively, what this indicates to me is that depending on the year, environment, soil type and weed spectrum you have, either of these approaches might work for you, but year-in and year-out, the two-pass pre-emergence followed by a postemergence herbicide program should provide the highest levels of weed control and corn yields.”

The idea of farmers rotating from Roundup Ready cotton or Roundup Ready soybeans to Roundup Ready corn does raise concerns about the potential for increased selection pressure on problem weeds.

“More than likely, if farmers are planting soybeans — or cotton — they are planting Roundup Ready varieties and treating with glyphosate at least once per season,” he said. “If we start planting more Roundup Ready corn, many fields will be in a Roundup Ready corn/Roundup Ready soybean rotation where glyphosate will be utilized as the primary, if not the only, postemergence herbicide.

“This will greatly increase the likelihood of selecting for a glyphosate-resistant weed, even in a corn-soybean rotation, because we are still spraying the same herbicide — glyphosate — in the same place over time.”

One alternative would be to use conventional herbicides in a conventional corn/Roundup Ready soybean rotation, says Bradley. “To my knowledge, no glyphosate-resistant weeds have been selected for in this kind of crop rotation and herbicide system.”

Another approach would be to apply pre-emergence herbicides in Roundup Ready corn, a practice Bradley considers a necessity to prevent early season yield loss. “I believe postemergence-only herbicide programs in Roundup Ready corn will cost you money,” he said.

“Alternative postemergence herbicides are another consideration, but if a grower has paid the tech fee for the Roundup Ready technology, more than likely, glyphosate will be the postemergence herbicide of choice.”

Bradley says growers should be cautious about applying one-half to two-thirds of the recommended pre-emergence herbicide rate, a practice that was being talked up by some industry representatives over the winter.

“Using a half-rate of a pre-emergence herbicide is almost like having a planned failure,” he said. “You will almost certainly have to come back with glyphosate in Roundup Ready corn, so the selection pressure on problem weeds will be that much greater.”

A number of atrazine-containing herbicide mixtures — Bicep, Guardsman Max, and Harness Extra, among others — are available to corn producers. Some three-way mixtures, such as Lexar, which provide a broader spectrum of control in corn, are also being marketed.

Farmers in Missouri, Iowa, Illinois and eastern Kansas, in particular, are having to take extra steps to control glyphosate-resistant waterhemp, a distant cousin to Palmer amaranth, a weed that is becoming all too familiar to Southern producers.

As growers get into the season, they need to pay special attention to the label restrictions on postemergence herbicides, particularly in conventional corn. Those include height and growth stage requirements.

“Atrazine-containing herbicides may be applied postemergence up to a height of 12 inches in corn,” he said. “Most of the ALS inhibitors, such as Steadfast, can be applied up to 20 inches or six collars, whichever comes first.

“Each year, I get a call from someone who waited too late to apply the ALS inhibitors. That can lead to yield losses through ear pinching — they interfere with ear formation, which can lead to lower yields.”